The majority of today's millionaires are self-made and weren't born into their wealth, new research shows.

The study by Fidelity Investments found that 86 percent of today's millionaires did not consider themselves wealthy growing up. Overall, the research revealed current millionaires are, on average, 61 years old with $3.05 million in assets.

While nearly three-quarters of millionaires feel rich, those who do not said they would need an average of $5 million of investable assets to begin feeling wealthy.

"Today’s millionaires are multi-dimensional, and to really understand them, you need to look not only at their outlook, but also at their path to wealth and their financial goals for the future," said Sanjiv Mirchandani, president of National Financial, a Fidelity Investments company.

Of those who are self-made millionaires, the study revealed their top sources of assets included investments/capital appreciation, compensation and employee stock options/profit-sharing. Those who were born wealthy were more likely to cite inheritance, entrepreneurship and real estate investment appreciation as an asset source.

When it comes to investment strategies, those who are self-made millionaires were more likely to add equity investments, while those who were born wealthy typically had more real estate investments.

The research shows that when considering their financial future, 30 percent of today's millionaires were concerned with preserving their wealth, while 20 percent were focused on growing their fortune. [The Best Age to Become a Boss]

This year’s study found that millionaires' outlook of the future financial environment continues to improve, with their optimism reaching the highest level since the survey’s inception in 2006.

"One trend has held true throughout the life of this study – the millionaire investor’s outlook has been consistently pragmatic about current market conditions and pervasively optimistic about a future recovery,” said Michael R. Durbin, president of Fidelity Institutional Wealth Services. "In many ways, what millionaires have been thinking and doing can be a strong indicator for financial trends."

Once such trend is millionaires' current interest in the stock market. The millionaires surveyed ranked individual domestic stocks as their top investment added in the last year, followed by certificates of deposit/money market accounts/cash equivalents, equity exchange traded funds, individual domestic bonds and domestic equity mutual funds.

The study was based on surveys of more than 1,000 millionaire investors.